A locking device which can be unlocked by means of a radio key may provided for a transmitting device seeking or stimulating the radio key in the surroundings of the motor vehicle by means of a request signal and then authorizing the key with the aid of a response signal from the radio key. With the request signal and/or the response signal, predetermined data and/or secrets are exchanged via a radio interface. Due to the limited transmission ranges of the request signal and the response signal, as well as an optional locating of the radio key, the use of the radio key is confined to a predetermined surrounding area bordering on the motor vehicle. Outside the surrounding area, the radio key does not work.
In a so-called relay attack on the locking device, for example to steal the motor vehicle, the request signal of the locking device is detected in the surroundings of the motor vehicle and extended over a larger distance than the mentioned surrounding area. For this, a theft device may be utilized, having a part near the vehicle and a part near the key. By means of the part near the vehicle, the request signal of the locking device is received and transmitted by means of radio communication, such as via a WLAN connection (WLAN—Wireless Local Area Network) or a mobile radio connection to the part near the key. This then sends out the request signal once again in the original frequency region. Thus, the surrounding area is enlarged beyond the originally defined boundaries of the surrounding area. If the radio key then answers with its response signal, this response signal will again be transmitted by the part of the theft device near the key via the mentioned radio communication to the part near the vehicle and here it is once more sent out in the original frequency region of the response signal to the motor vehicle. In this way, the motor vehicle can respond in the same way as if the radio key were operating immediately in the surrounding area itself. Hence, accessing and starting of the motor vehicle is then also possible if the radio key is located outside the surrounding area, i.e., in a dwelling, for example.
A theft device which can be operated in the described manner is known for example from DE 10 2014 010 668 A1. The radio key and the locking device in this case transmit their signals at a level at which a signal change occurs in a special pattern.
From WO 2015/013087 A1 it is known how to use time-changing frequencies for the transmitting of the request signal and/or the response signal in a locking device.
It is known from DE 101 55 123 A1 that a locking device can recognize a radio link between said part of a theft device near the vehicle and the part near the key and then not release the vehicle. For this, a determination is made as to whether a signal received outside a useful frequency band is permanently present or whether it is a manipulated code signal.